The present invention relates to a display apparatus such as a plasma display panel (PDP) system or a digital micro-mirror device (DMD), and more particularly, to a display apparatus in which one television field is divided into a plurality of sub-fields to get a variety of gray scale intensity.
Among flat panel displays, a PDP system is the easiest to construct into a larger size and exhibits excellent fundamental performances such as response time, color reproducibility or the like, and is expected to be the most promising candidate for a wall-mounted television set.
In the known PDP system, a period for one field is divided into a plurality of sub-fields, each of which is allocated a relative ratio of display time (i.e., luminescence time), which is chosen to be a power series of 2 such as 1:2:4:8: . . . so that a combination of luminescence and non-luminescence for the respective sub-fields provides a gradation display of each picture element.
FIG. 20 shows an example of luminescence sequence during one field. This figure shows the example in which one field is divided into eight sub-fields SF0 to SF7. A relative ratio of luminescence time of the respective sub-fields is chosen to be in the ratio of 1:2:4:8:16:32:64:128, and the combination of luminescence and non-luminescence of the individual sub-fields is capable of representing 256 gray levels.
For example, when a gray level of 127 is to be provided, the sub-fields SF0 to SF6 are in on-state while the sub-field SF7 is in off-state. A human eye has a time integrating effect and does not respond to on/off of luminescence within one field. Thus, the luminescence from the sub-fields SF0 to SF6 are integrated by a human eye, providing a perception as if a gray level of 127 has been given.
When a video signal is to be displayed by the display apparatus, the video signal is initially converted into an 8-bit digital signal. The least significant bit b0 is assigned to a sub-field SF0, the second least significant bit b1 to a sub-field SF1, the third least significant bit b2 to a sub-field SF2, . . . , and finally the most significant bit b7 is assigned to a sub-field SF7.
FIG. 21 is a block diagram showing a prior art display apparatus which implements a gradation display. As shown in FIG. 21, the display apparatus has an input terminal 1 to which a video signal is input; an input terminal 2 to which sync signals are input; an A/D converter 3 in which the video signal input to the input terminal 1 is converted into a digital signal; a field memory 4 which stores two fields of output signal from the A/D converter 3; a driver 5 which drives a PDP 7 in accordance with output signals from the field memory 4; the controller 6, which controls the A/D converter 3, the field memory 4 and the driver 5 on the basis of the sync signals; and the PDP 7.
Next, the operation will be described. The video signal which is supplied from the input terminal 1 is converted into an 8-bit digital signal in the A/D converter 3, and two fields of digital signal are stored in the field memory 4. The field memory 4 includes a pair of field memory sections, and input signal is alternately written into the first field memory section and the second field memory section.
Next, during an address period of the sub-field SF0 shown in FIG. 20, the controller 6 controls the field memory 4 so that the data for a bit b0 is read from the field memory 4. At this time, the data is read out of either memory section to which a write operation is not being made. Data read is passed through the driver 5 to be written into the PDP 7. For an AC plasma display, the panel has an inherent memory which allows written data to be maintained during a period required for data for the whole screen to be written into the PDP 7. During a subsequent sustain period, the controller 6 controls the driver 5 so that luminescence from the PDP 7 occurs only from a picture element for which data for the bit b0 is set to be in on-state.
During a subsequent address period of the sub-field SF1, data for a bit b1 is read from the field memory 4 and fed through the driver 5 to the PDP 7. During a subsequent sustain period of the sub-field SF1, luminescence occurs for a period which is twice as long as the sustain period of the sub-field SF0.
Similarly, during the sub-fields SF2 to SF7, the corresponding bits b2 to b7 are read from the field memory 4 during the respective address periods and fed through the driver 5 to the PDP 7, allowing luminescence during the respective following sustain period for respective periods which are 4, 8, . . . , 128 times longer than the luminescence time in the sub-field SF0.
In the display apparatus which provides a gradation display in the manner mentioned above, it occurs that when a flat image which varies smoothly in the horizontal direction moves horizontally across the screen, a vertical strip-shaped band which was invisible when the image was at rest appears to be perceived, such band being hereafter referred to as a dynamic false contour. Similarly, when a flat image which smoothly varies in the vertical direction moves vertically across the screen, a dynamic false contour is again perceived. This phenomenon will be further described with reference to FIG. 22 and FIG. 23.
FIG. 22 illustrates that an image which varies smoothly in the horizontal direction, namely an image having a gray level which changes from 127 to 128, is moving to the left at the rate of two picture elements per field.
When representing a gray level of 127, luminescence occurs for seven sub-fields including SF0 to SF6, and when representing a gray level of 128, the luminescence occurs only for the sub-field SF7. When such an image is viewed by a human being, the line of vision is roughly indicated by broken lines R.sub.0, R.sub.1 and R.sub.2. A position on the retina which corresponds to a region to the left of the broken line R.sub.0 will perceive a gray level of 127, while a position on the retina which corresponds to a region located to the right of the broken line R.sub.2 will perceive a gray level of 128. However, a position on the retina which corresponds to the broken line R.sub.1 will perceive substantially null, which is perceived as the false contour. FIG. 23 is a diagram showing a relationship between relative perception quantity of brightness and a position on the retina.
There is a tendency that such a phenomenon is readily perceivable upon movement of an image containing a change from the gray level of 127 in which seven sub-fields SF0 to SF6 are turned on to the gray level of 128 in which the luminescence occurs only during the sub-field SF7, namely, an image having an up-shift from a lower significant bit to the most significant bit, or a down-shift from the most significant bit to the lower significant bit. This is attributable to two points described below.
1) Between the adjacent gray levels, there is a significant barycenter shift in the luminescence time within one field. Namely, for the gray level of 127, the luminescence occurs early within one field in a concentrated manner, while for the gray level of 128, the luminescence occurs late within one field in a concentrated manner. PA1 2) Between the adjacent gray levels, the magnitude of change in the amount of luminescence from non-luminescence to luminescence or from luminescence to non-luminescence is large. Specifically, sub-fields, which are turned on at a gray level of 127, is turned off at the gray level of 128, while the sub-field, which is turned off at a gray level of 127, is turned on at the gray level of 128.
FIG. 24 is a diagram showing a gradation display method used for a prior art display apparatus as disclosed in Japanese Patent Kokai Publication No.211,294/1992. Specifically, a sub-field which corresponds to the most significant bit b7 is evenly divided into sub-fields SF7-1 and SF7-2, placing the luminescence time regions at both the beginning and the end of one field.
By using such a luminescence sequence, the perception quantity of a false contour can be reduced. This will be described with reference to FIG. 25 and FIG. 26.
FIG. 25 illustrates a case in which an image, having a change of the gray level from 127 to 128 in the same way as FIG. 22, moves to the left at the rate of two picture elements per field. At the gray level of 127, seven sub-fields SF0 to SF6 are turned on, while at the gray level of 128, only sub-fields SF7-1 and SF7-2 are turned on.
When such an image is viewed by a human being, the line of vision is roughly indicated by broken lines R.sub.0, R.sub.1, R.sub.2 and R.sub.3. A position on the retina which corresponds to a region located to the left of the broken line R.sub.0 will perceive a gray level of 127, while a position on the retina which corresponds to a region located to the right of the broken line R.sub.3 will perceive a gray level of 128. Further, a position on the retina which corresponds to the broken line R.sub.1 will perceive a gray level of about 191, and a position corresponding to the broken line R.sub.2 will perceive a gray level of about 64. A relationship between the relative perception quantity of brightness and the position on the retina is shown in FIG. 26. It will be apparent that an improvement is achieved over the example described above.
Although it will be apparent that such the prior art display apparatus (shown in FIG. 24 and FIG. 25) is constructed in the above described manner and is improved over the above example shown in FIG. 20 and FIG. 21, such improvement is insufficient for use with actual images.
This is because while a barycenter shift in the luminescence time between the adjacent gray levels is reduced, the magnitude of change in the amount of luminescence which occurs between luminescence and non-luminescence across the adjacent gray levels remain to be large in the same manner as the above described example (FIG. 20 and FIG. 21), thereby reducing the improvement.
As indicated above, there is a problem that a false contour which is invisible when the image is at rest becomes perceivable when a flat image which varies smoothly moves across the screen.